Mechanical vs. Vacuum carbs
Thread Starter
Instructor
Joined: Jul 2005
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Mechanical vs. Vacuum carbs
For the 383 crate engine that I'm going to buy, the vendor is suggesting an 850 CFM mechanical secondaries carburetor.
From what I've heard, vacuum carbs are much easier to keep tuned.
What is the advantage of a mechanical carb over a vacuum one? Do they give better horsepower? I dont want to have to tweak with the carb all the time to keep it properly set.
Also, would 850 CFM be overkill for a 383? Would an 850 be bad for fuel consumption, compared to a 750cfm?
From what I've heard, vacuum carbs are much easier to keep tuned.
What is the advantage of a mechanical carb over a vacuum one? Do they give better horsepower? I dont want to have to tweak with the carb all the time to keep it properly set.
Also, would 850 CFM be overkill for a 383? Would an 850 be bad for fuel consumption, compared to a 750cfm?
Advanced
Joined: May 2006
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From: Derby Kansas
850 sounds like an over-kill, but with no additional info provided, it would be difficult for anyone to provide a confident response. Need more info, such as, anticipated hp, exhaust, cam, trans, rearend, driving habits (street,strip, etc..., what is your goal.
-Jim
-Jim
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450HP/475TQ
Patriot aluminum heads
Cam Lift: 543/545
Cam Duration: 234/240
Transmission: th 350
Exhaust: Dual exhaust. It's not the stock one, but I dont have any details on it. It was on the car when I bought it.
Use: Mostly street, with some track
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From: Lakeland, FL
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St. Jude Donor '03, '06, '17
I agree that 850 cfm is overkill but I would suggest to go with a mechanical secondaries carb.. I gained big HP on my old 383 when switching from vacuum secondaries to mechanical secondaries. The vacuum secondaries never opened up 100%. This could've been fixed by changing springs in the vacuum secondaries, but I prefer to run mechanical secondaries as I know that my secondaries are always open at WOT!
Safety Car
Joined: Oct 1999
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From: NorCal
850 is a bit much, and I'd go with a vacuum secondary with an automatic street ride. You'll get a much smoother transition into the secondaries (that's the advantage of vacuum secondaries) and you'll probably be happier with the way it works with the transmission kick-down/downshift. If you go with a Holley style carb (BG, Holley, Demon), get one with the jet access plugs in the front of the bowl. That way you can easily adjust the jetting once it's running. The AFB style carb (Edelbrock) is tuneable as well. I'd go for around 750 cfm to start.
Hans
Hans
Drifting
Joined: Nov 2002
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From: Loveland, CO
I agree with FB007. If you want a street car use vacuum secondaries. If you want a race car, use mechanical. That is also the recommendation of Dave Emanuel in his book on Holley Carburetors.
Everybody will have a differing opinion.
Ralph.
Everybody will have a differing opinion.
Ralph.
Racer
Joined: Jan 2008
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From: Arvada Colorado
There is a lot of truth and fantasy about vacuum secondaries. I vintage road race a small block using vacuum secondaries. I called up Holley and discussed this at length with them. Certainly, I want instant acceleration off a corner and cannot accept any lag or lack of power. Holley likes the vacuum secondaries for this application because it is "tunable". Yes, you need to play with springs in the vacuum canister and yes, you need to tune the accelerator pump with cams and different squirter sizes. There are other things in the equation too, with power valves, ignition advance curves, cams etc, etc.
Out of the box, a Holley with vacuum secondaries, probably will not work well if you do not change canister springs. Altitude plays a big factor too.
Out of the box, a Holley with vacuum secondaries, probably will not work well if you do not change canister springs. Altitude plays a big factor too.
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Thread Starter
Instructor
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When you say performance, do you mean horsepower, throttle responsiveness, both, something more?
If horsepower, how much HP would I be leaving on the table if I went with vacuum instead of mechanical?
Why would one make more horsepower than the other, if they both flow the same cfm?
Last edited by pj_corvette; Jan 16, 2008 at 11:42 AM.
Racer
Joined: Jan 2008
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From: Arvada Colorado
You are correct. If flow is the same, jets, etc same. all are equal. Vacuum secondaries are constructed a little different than mechanical such that the secondary fuel system is a bit different. Aside from that, it's equal.
Burning Brakes
Joined: Mar 2004
Posts: 816
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From: Finland
I agree that 850 cfm is overkill but I would suggest to go with a mechanical secondaries carb.. I gained big HP on my old 383 when switching from vacuum secondaries to mechanical secondaries. The vacuum secondaries never opened up 100%. This could've been fixed by changing springs in the vacuum secondaries, but I prefer to run mechanical secondaries as I know that my secondaries are always open at WOT!
go with 750 mechanical
Safety Car
Joined: Oct 1999
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From: NorCal
There is a lot of truth and fantasy about vacuum secondaries. I vintage road race a small block using vacuum secondaries. I called up Holley and discussed this at length with them. Certainly, I want instant acceleration off a corner and cannot accept any lag or lack of power.
Now for drag racing I ran a dual feed, double pump mecanical secondary 850 cfm on a RA IV Pontiac engine in a '64 GTO automatic that carried the front wheels running 12.40's on cheater slicks. THAT was instant power!
Hans
Racer
Joined: Sep 2006
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From: McComb Ms.
To find your carb size i think its hp x redline rpm divided by 3456 = carb cfm...for instance, my ZZ4 crate is 335hp and 5800 redline rpm. so 355hp X 5800rpm divided by 3456 =595.7 , so i'd want a 600 cfm. GM recommended a 780 cfm when i purchased the motor in 1999. I don't know the deal on that.
I have a mechanic friend who told me he could convert a vacuum carb to mechanical simply by putting a screw in the linkage. to the secondary making it open upon throttle. Maybe that will help
I have a mechanic friend who told me he could convert a vacuum carb to mechanical simply by putting a screw in the linkage. to the secondary making it open upon throttle. Maybe that will help
Racer
Joined: Jan 2008
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From: Arvada Colorado
Wrencher
Perhaps I spoke a little hastily when discussing power out of the corner and Road racing. You are correct in the fact that out of a corner it is "the judicious application of power" that gets the job done. However, I don't think that the carb is the governing factor but your right foot. As you know, your engine only takes in what it it can handle.
I put the quote in parenthesis as there is a gentleman that runs with us that used to drive in the Trans-AM series ( years ago ) that runs a Boss 302. The man builds Mustangs for a living and is very well respected. He used that quote not me.
Perhaps I spoke a little hastily when discussing power out of the corner and Road racing. You are correct in the fact that out of a corner it is "the judicious application of power" that gets the job done. However, I don't think that the carb is the governing factor but your right foot. As you know, your engine only takes in what it it can handle.
I put the quote in parenthesis as there is a gentleman that runs with us that used to drive in the Trans-AM series ( years ago ) that runs a Boss 302. The man builds Mustangs for a living and is very well respected. He used that quote not me.
Team Owner
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From: All humans are vermin in the eyes of Guru VA
Cruise-In IV Veteran
Cruise-In V Veteran
For some odd reason, I have been thinking that individual runner intakes require more carbueration than do dual planes, and Dual planes require more carbeuration than do single plane intakes.
Drifting
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From: Cameron Park CA
Cheers,
Pete
Melting Slicks
Joined: Jan 2004
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From: Detroit MI
Get a new mechanic friend.Quote from Holley:
DO NOT put a screw in the linkage of a vacuum secondary carburetor to mechanically “force” open the secondary throttle plates.
Normally there is an accelerator pump on the secondary side of a mechanical secondary carburetor. The purpose of the secondary
pump is to inject additional fuel to “cover” the transition time up to the point when the secondary main system starts to flow. Without
this secondary pump shot the engine will go to an instant lean condition. Therefore, forcing the secondaries to open prematurely
will hinder performance and may cause an engine backfire. The screw could also create a bind and cause the throttle to stick open.
Last edited by 69autoXr; Jan 16, 2008 at 11:16 PM.
